The effect of an external disk on the orbital elements of a central binary

Using smoothed particle hydrodynamics, the quasi-stationary structure of a circumbinary disk is found and its gravitational interaction with the central binary system is evaluated. The resulting rate of change of the orbital elements of the binary is examined. It is demonstrated that, in general, the semimajor axis decreases, the eccentricity grows, and the orbit precesses in the prograde sense. Numerical results for one particular model and a qualitative explanation for the effects found, mainly that of eccentricity driving, are provided. A protobinary or newly formed binary star embedded in a disk may undergo a significant orbital evolution before the disk dispersal. The application of the results to the interpretation of the observed statistics of binary eccentricities is discussed.